Individual drives cannot reliably be used, in practice, as backup drives. In theory, they can and this is apparently Seagate's position as mentioned below, but as you look at actual backup practice, there are a number of classes of failure which are weaknesses in the single-backup-drive environment. Backup drive hardware failure represents one class of failures. Backup drive firmware failure might represent another, rare, class of failures. OS failures (bugs) and incompatibilities, backup software failures and incompatibilities, and user error, each represent additional classes of possible failure.
One Seagate chat-based tech support apparently believes that a backup drive which has failed is not too serious, because, afterall, it is a BACKUP DRIVE -- meaning the live data is still intact, and it's just that the backup has failed so there is no actual data loss. (Can you believe it?) The possibility that data may have been migrated or that the (inaccessible) backup drive is now being called upon to do a restore, seems not to have occurred to them. (Can you believe it?)
As to backup drive failure classes, of four Maxtor drives we have -- 300 GB, 500GB, 1TB and 1.5TB -- three of them have failed in one way or another. One was an outright hardware failure, one a "false" Maxtor diagnostic error which Seagate tells us to ignore (?), and one a completely inaccessible drive possibly brought about by backup software overrun (1 TB drive out of space!) or maybe the loose thing rattling around inside the case.
In any events, a single external drive unit as backup is in practice a risky thing. The MTBF figures are suspect enough that at least one offsite and one onsite ext drive should be used, paralleling the on- and off-site backup tape storeage used in the old days.
Didrik